Method for removal of electrical shorts in a sputtering system

ABSTRACT

Techniques for removing an electrical short caused by a flake in a thin film sputtering system are disclosed. The flake typically bridges the dark space between target and anode or shield and is removed through manipulation of the power supply utilized for normal operations. Sensing of the flake and discrimination between recoverable arcs is accomplished by timing the duration of an over-current condition. Removal involves switching between power mode or some other initial mode of regulation and current modes and progressively increasing current to melt the flake. Circuitry automatically removes the flake and is easily adapted to power supplies particularly more sophisticated, high frequency, lower energy storage DC supplies.

BACKGROUND OF THE INVENTION

Generally the invention relates to the field known as thin filmprocessing through sputter techniques. More specifically, the inventionfocuses on the undesirable aspect of electrical shorts during thesputter process, presenting both methods and apparatuses for removingsuch electrical shorts upon their occurrence.

The sputtering process in general is well known in the art. It wasapparently first reported in 1852 by Sir William Robert Groves. In 1921Joseph John Thompson initially named the process "spluttering"; laterthe "1" was dropped and the name for the process became "sputtering."This term is meant to describe a process whereby atoms of a material aremechanically freed from a surface through a momentum transfer. The atomsthen dissipate to cause a thin film on or interact with a surface orsubstrate. Although the sputtering process has been known for some time,in recent years applications of the process have grown dramatically andhave been subject to refinement and development with technologicaladvances. To a significant degree the increase in focus on thesputtering process has been due to the growth of the semiconductorindustry which has increased the focus on thin film processes in recentyears and has made available more sophisticated equipment to practicethe process.

To properly understand the problem addressed through the presentinvention, it is necessary to generally understand the operation of thesputter process. An excellent discussion of the process is contained inthe textbook Glow Discharge Processes by Brian Chapman published in 1980by John Wiley & Sons, Inc. As relevant to the present invention, thesputter process is a process whereby the surface of an item is coated(deposition), removed (etched), or whereby the surface of an item isconditioned. In basic form, a sputtering system involves a power supplywhich ionizes a gas. This ionized gas or plasma then accelerates to atarget which contains the material that will become the coating. Whenthe ions strike the target, atoms of the target are released throughmomentum transfer. These atoms then dissipate and some eventuallycontact a substrate and become a coating on that substrate. Although itis a goal to achieve as uniform a process as possible, inherent noiseand anomalies in both the plasma generation and the material of thetarget can cause fluctuations in the process. These fluctuations canresult in the release of particles or flakes from the target rather thanthe individual atoms desired. In addition--and perhaps to a moresignificant degree--the coating itself tends to peel off of all surfacesin the chamber. When it peels off of any surface, flakes are createdwhich fall or lodge across the power supply. Also, the coating can buildup in undesirable places. This results in the same effects as a flakeand is thus referred to synonymously as such. In particular targetgeometries, occasionally one of these flakes occurs across the powersupply causing an electrical short. Such a short is not only undesirablebut they are very difficult to predict because the entire environment isvery dynamic.

The significance of these flakes for the particular target geometriesinvolved is a function of two factors: the energy capacity of the powersupply and the particular target material involved. In prior years itwas common to utilize power supplies which rectified relatively lowfrequency line sources and which had high energy capacities relative tothe amount of energy necessary to melt the types of flakes which are thefocus of the present invention. In recent years it has become understoodby those skilled in the art that higher frequency, lower capacity powersupplies provide better process control for reasons unrelated to theoccurrence of flakes. Although the older, high energy capacity powersupplies easily release sufficient energy into some flakes to not justmelt, but to vaporize the flake, larger flakes exist which are notmelted during normal processing. When the more sophisticated, lowerenergy capacity power supplies are utilized, however, the flake problembecomes more acute. Not only are the larger flakes not melted, but thesmaller flakes now become a problem as well. These lower energy storagepower supplies simply react too quickly and have insufficient storedenergy to melt even the smaller flakes which occur. Prior to the presentinvention, when a flake did occur for these more sophisticated powersupplies, an over current condition would automatically shut down thepower supply. Those skilled in the art then would manually shut down theentire sputter system and would physically remove the flake from thetarget surface. This not only necessitated long time delays but it alsocaused an unevenness in the treatment of the substrate. The presentinvention addresses this problem as it relates to both the older, higherenergy storage power supplies and as it relates to the moresophisticated, lower energy storage power supplies. Since the problem ismore acute in the lower energy storage power supplies, however they arethe focus of this disclosure.

As mentioned, another factor involved in the occurrence of flakes is thenature of the particular target involved. Although almost all targetsare commercially manufactured to minimize the possibility of anoccurrence of a flake or impurity being released, some materials aremore prone to this than others. For instance the invention in U.S. Pat.No. 4,610,775 to Phifer is directed to the occurrence of flakes whencoating nuclear fuel pellets with a thick layer of zirconium diboride(ZrB₂). Apparently this material has a tendency to produce flakes. ThePhifer disclosure patents the solution of providing a separate AC powersupply, which provides 150 to 200 amps at 60 volts AC to melt thoseflakes. In sharp contrast, the present invention provides a simpler,less expensive solution which utilizes the existing power supply to meltthe flakes. While the problem of electrical shorts caused by flakes hasbeen known, the fact that the flakes shorted the power supply utilizedin normal operation led those skilled in the art away from utilizing itas a solution to the problem. Since the flake shut down the powersupply, it was felt that an external solution was required. Also insharp contrast, the present invention provides only sufficiently enoughenergy to melt the flake, rather than to vaporize it. This avoidsuncontrolled sputtering and provides a more uniform end result thansolutions which may vaporize the flake. This is because "vaporizing" theflake results in it exploding and splattering material inside thesystem.

Similarly, inventions such as discussed in U.S. Pat. Nos. 3,544,913 and3,546,606 to Anderson involve a totally different environment. Asmentioned, those inventions relate to the field of electron beamprocessing in which an arc discharge is "starved" before development byquickly limiting the current which creates the electron beam. Becausethose inventions involve arcs rather than flakes, the clearing orremoval process is fundamentally different. The electrical short is not"starved" in the present invention, rather it is melted by driving morecurrent through it.

SUMMARY OF THE INVENTION

The present invention is directed to methods and apparatuses forremoving electrical shorts in sputtering systems. The methods addressthe problem of electrical shorts by providing a solution which isadaptable to both high and low energy storage power supplies as now usedin many sputtering applications. It is an object of the invention toprovide an automatic removal process, thus eliminating the need forfrequent human intervention in the process. It is also an object of thepresent invention to provide the ability to remove most electricalshorts caused by flakes in a sputtering system in as rapid a time aspossible. Thus it is also an object of the present invention to removethe flake and to then return to normal sputter processing as quickly aspossible.

In addition, the present invention addresses the problem of electricalshorts in a sputtering system by manipulating the power supply used innormal operation to affect the removal phase. An object of the presentinvention is thus to affect removal through the single power supplyinvolved in a simplified sputtering system. Accordingly it is an objectof the present invention to provide the capability of removing mostflakes without adding another power supply.

It is an object of the present invention to affect the removal byprogressively increasing the application of current or by increasingcurrent in a controlled manner. In one embodiment, it is also an objectof the invention to provide a circuit through which the progressiveincrease in current is a linear increase.

It is also an object of the present invention to avoid uncontrolledsputtering by avoiding vaporizing, splattering, or exploding any flakeswhich occur. An object is thus to provide only sufficiently enoughenergy to remove the flake and to avoid supplying excess energy into theflake removal process.

It is a further object of the present invention to provide a circuit forswitching the regulation mode of the power supply to thus affectimmediate removal of most electrical shorts.

An important goal of the present invention is to provide a solution tothe problem of flakes which minimizes the amount of equipment involved.An object is thus to provide as simple and as inexpensive a solution aspossible.

It is also an object of the present invention to provide for methodswhich may be controlled through hardware or software configurations. Anobject of the present invention is to provide general techniques whichmay be readily adapted to a hardwired circuit such as disclosed herein,to a programmable processor as might be internally adapted to control apower supply, or to a computer system as might be externally utilized tocontrol one or more power supplies.

It is still a further object of the invention to provide methods for theremoval of an electrical short which may be easily adapted to existingpower supplies and existing sputtering systems. This includes both DCand AC power supplies as are commonly utilized in a sputtering system.

Another object of the present invention is to present a technique andapparatus to automatically sense an electrical short or flake and todistinguish it from other occurrences of recoverable arcs or sparks. Thepresent invention presents methods which can both minimize thepossibility of an erroneous detection and which can detect theoccurrence of a flake in as short a time as possible.

Naturally, further objects of the invention are disclosed throughoutother areas of the specification and claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a diagram of a basic sputtering system.

FIG. 1B is a diagram for explaining the formation short circuits in thesputtering system of FIG. 1A due to flakes.

FIG. 2 is a graphic representation of the general behavior of thecurrent which would be supplied by the power supply upon an occurrenceof an electrical short caused by a smaller flake and its removal.

FIG. 3 is a graphic representation of the general behavior of theimpedance across the power supply during the time corresponding to thatshown in FIG. 2 upon an occurrence of an electrical short caused by asmaller flake and its removal.

FIG. 4 is a graphic representation of the general behavior of thecurrent supplied by the power supply upon an occurrence of an electricalshort caused by a larger flake and its removal.

FIG. 5 is a graphic representation of the general behavior of theimpedance across the power supply during the time corresponding to thatshown in FIG. 4 upon an occurrence of an electrical short caused by alarger flake and its removal.

FIG. 6 is an expanded block diagram of a lower energy storage powersupply modified to practice the present invention as might be connectedto function in a sputtering system.

FIG. 7 is an expanded block diagram of the flake removal circuitry shownas a component in block form in FIG. 6.

FIG. 8 is a schematic diagram of the flake removal circuitry shown inblock form in FIG. 7.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As mentioned earlier, it is necessary to understand the generalsputtering process in order to understand the scope and application ofthe present invention. FIG. 1 shows a cross section of a simplifiedsputtering system. The process occurs in a chamber (1) which isevacuated by vacuum pump (2). Within chamber (1) is placed a target (3)which will become the coating itself. In the particular targetgeometries involved, target material (3) is surrounded by anode (4) alsowithin chamber (1). Anode (4) may serve as a shield as referred to inthe literature when it is connected to ground. Anode (4) may, however,be electrically isolated from other shields within chamber (1). Chamber(1) also contains substrate (5) to be coated. In operation biaspotential is maintained between target (3) and anode (4) through theconnection of power supply (6). When sufficient vacuum is present withinchamber (1), sputtering gas (7) is introduced in chamber (1). Sputteringgas (7) (typically an inert gas) is ionized by operation of power supply(6) and results in plasma cloud (8) being formed in a region adjacent totarget (3). Plasma cloud (8) is maintained in equilibrium through thecontinuous operation of power supply (6). Power supply (6) not onlyionizes the gas, but also attracts the ions to target (3). Though theresultant collision of ions into target (3), momentum is transferredfrom the ions to the surface of target (3), resulting in release ofmostly uncharged atoms of the material of target (3). These atoms thenfreely dissipate and some strike substrate (5) and result in a coatingon substrate (5).

In the particular type of sputtering system shown in FIG. 1, a specific,commercially available target assembly is involved. This target assemblyis frequently characterized by a circular disk which becomes target (3)and which is surrounded along its periphery and backside by anelectrically conducting anode (4). Although many different geometriesare possible in various sputtering systems, with respect to the presentinvention an important characteristic of this type of target assembly isthat the space between target (3) and anode (4), gap (9), is very small.Gap (9) is purposely kept small. Because of the physics involved and thepurpose of the anode to allow creation of plasma only in the directionin which it is desired to sputter atoms, the distance of gap (9) isfrequently sufficiently small to be less than the distance of dark space(10). Dark space (10) refers to the region close to target (3) andbetween plasma cloud (8) in which plasma does not exist. Because of thephysics of the generation of plasma cloud (8), by maintaining gap (9)less than the distance across dark space (10), no plasma (and thus nosputtering) occurs between anode (4) and target (3). The technique ofproviding a small gap between target (3) and anode (4) is well known inthe art and is discussed in the textbook Glow Discharge Processes byBrian Chapman referred to earlier.

Because of the physical necessity to keep gap (9) sufficiently small,occasionally particles or flakes may be released from target (3), maygrow on shield (4), or may be released from other surfaces in chamber(1) which may be of sufficient size to bridge gap (9), thus causing anelectrical short between target (3) and anode (4). This electrical shortis therefore across power supply (6). Again, this occurrence of flakesand the release of particles from target (3) and other surfaces inchamber (1) is well known in the art. An excellent discussion of thereasons for the occurrence of flakes is available in the textbookDeposition Technologies for Films and Coatings by Rointan F. Bunshah etal., published in 1982 by Noyes Publications. Referring back to FIG. 1,in the expanded view involving gap (9), it can be seen that there isshown a flake (11). Flake (11) completely bridges gap (9) and is thereason for the present invention. With the advent of more sophisticated,high frequency, lower energy storage power supplies the problem offlakes has become more acute. As is known by those skilled in the art,such power supplies offer advantages in the actual sputtering process.One disadvantage is that they are not capable of melting even smallerflakes under normal conditions. Thus, upon the occurrence of flake (11),power supply (6) would be shut down. Those skilled in the art would thenphysically break vacuum and manually remove flake (11) through wirebrushing or other means or were forced to utilize an auxiliary powersupply as in U.S. Pat. No. 4,610,775. Although the benefits by use ofthe higher frequency, low energy storage power supplies greatlyoutweighed the determent of having to address even smaller flakes, thisdrawback has long been recognized although prior to the presentinvention no solution utilizing the existing power supply has beenavailable.

In the present embodiment, power supply (6) is shown as a regulated DCpower supply adapted to a power regulation mode. This involves voltagesin the range of approximately 500 to 1500 volts and currents rangingfrom 10 to 20 amps. Such power supplies are well known and readilyavailable to those skilled in the art. In addition, AC power suppliesmay be utilized. Naturally, the techniques and methods of the presentinvention may be adapted to such equipment as well.

As mentioned earlier, the problem which is addressed by the presentinvention exists for both the older, low frequency power supplies andthe more sophisticated, higher frequency power supplies. It is moreacute, however, for the latter. The characteristics of such asophisticated, high frequency, power supply as they relate to sputteringare well known and are specifically discussed in the technical bulletinsentitled "The MDX as a Strategic tool in Reducing Arcing" by Douglas S.Schatz first published in 1983 and "Arcing Problems Encountered DuringSputter Deposition of Aluminum" by Thomas C. Grove published in 1986.Among the relevant characteristics, these power supplies (to which thepresent invention would be particularly important) would include lowerenergy storage, a regulation capability, a feedback loop and controlcircuitry as discussed later. While the present invention would notcompletely eliminate the problem of flakes, it would minimize it in thelower energy power supplies and would enhance it in the higher energypower supplies.

In operation, plasma cloud (8) operates to continuously release orsputter atoms from target (3). By its very nature, plasma cloud (8)operates somewhat randomly over target (3). Imperfections in target (3),ripple in the electrical potential supplied, and localized heating oftarget (3), among other factors, result in a degree of variability inthe amount of atomic material released. The material also coats allsurfaces in chamber (1). Many of these surfaces are not designed to havethe coating adhere to them. These effects result in undesirablevariations in the coating on substrate (5). These variations result inboth the occurrence of arcs across dark space (10) and flakes across gap(9). Although both appear as lower impedance or higher currentconditions, in fact over-current conditions, to power supply (6), thepresent invention must adequately distinguish between the two and reactdifferently to continue processing. The entire environment of chamber(1) is electrically very noisy; a rough equilibrium is maintained. Thisequilibrium is maintained in such a noisy environment through regulationof power supply (6). The occurrence of an arc across dark space (10) canbe quickly negated by short-term interruption of power supply (6) asknown in the art. For convenience, these types of over-currentconditions are thus referred to herein as "recoverable arcs." Thisinterruption need only last for a few milliseconds after which fullpower or voltage may be restored to power supply (6) as it existed priorto the occurrence of the recoverable arc with no interruption inregulation mode. The process can accommodate these anomalies and cancontinue uninterrupted after the short interruption with littleundesirable effect other than the loss of a few milliseconds. The secondsituation, the occurrence of flake (11) across gap (9), results in anelectrical short from which recovery by the techniques referred to aboveis not possible.

In order to solve the problems raised by the electrical short caused byflake (11), the present invention discloses both methods andapparatuses. Rather than incorporate a second power supply as was donein the Phifer invention disclosed in U.S. Pat. No. 4,610,775 discussedearlier, the present invention would remove the flake through operationof existing power supply (6). As set forth in the claims, the methodspresented herein involve several different combinations of steps.

Fundamentally the method involves two key steps. First the presence ofan electrical short or flake (11) must be detected and discriminatedfrom a recoverable arc. Although prior art had utilized a voltage orcurrent criteria, it is believed that within the ranges necessary forgeneral sputtering, an electrical short caused by a flake can be sensedby timing the duration during which power supply (6) is in anover-current condition. From application, it is believed that fortypical sputtering environments, if recovery has not been effected in aperiod of time ranging from about 10 milliseconds to 5 seconds, theprobability of recovery from the over-current condition, through mereinterruption in the operation of power supply (6) as utilized forrecoverable arcs is greatly reduced. In processing environments asdiscussed below, it is believed that any over-current condition whichlasts longer than 50 milliseconds is usually an electrical short causedby flake (11). In understanding the detection of flake (11) through thetechnique of timing the over-current condition, it must be recognizedthat since sputtering environments can vary, so too might the specificparameters involved. This time (50 milliseconds) also allows a marginwhich allows for the inherent variations in the process. Since theprocess parameters can vary widely, the broad range of from 10milliseconds to 5 seconds is deemed necessary to practically accommodatethe process parameters presently in use. Naturally the threshold timefor characterizing an overcurrent condition as unrecoverable or that ofan electrical short caused by a flake will vary in different electricaland configuration situations.

Referring to FIG. 2, the detection process can be graphically understoodthrough the behavior of the current output by power supply (6). Prior totime (A) power supply (6) operates by outputting a relatively constantnominal current (I_(nom)). This nominal current (I_(nom)) is in essencea measure of the amount of plasma cloud (8) being attracted to substrate(5). In referring to FIG. 2, it should be understood as referred tobefore that the environment within chamber (1) is electrically verynoisy. Some noise and variation is shown on the nominal current(I_(nom)), however, it is only shown in areas which are uninteresting,for simplicity. This is true for FIGS. 2, 3, 4 and 5. In addition, itshould be understood that each of these figures is designed forconceptual understanding only, time and value scales are not meant toimply any scaled relationship and are not linearly--or evenregularly--scaled. For instance, time (A) to (B) may be on the order ofmicroseconds, time (B) to (C) on the order of tens of milliseconds, andtime (C) to (E) on the order of hundreds of milliseconds.

As mentioned, power supply (6) operates nominally at times prior to time(A). At time (A) however, a flake occurs causing the current to rise.This current rise can be quite rapid and will eventually reach theover-current level (I_(oc)) triggering shut-off of power supply (6)through its own internal overcurrent protection circuitry. Becauserecoverable arcs exist, the power supply can shut off the current, waita brief time, and then attempt a restart in the prior regulation mode(usually a power regulation mode). If restart is not successful, thissequence may produce a series of pulses similar to that shownconceptually in FIG. 2. This over-current sequence of events occurs assoon as the current shown in FIG. 2 reaches the level (I_(oc)) at time(B). In instances of a recoverable arc, this pulsed process will negatethe over-current condition. In instances of an electrical short causedby flake (11), however, the pulsed process does not. Thus, in FIG. 2 acontinuous pulsed sequence of events occurs between the initialover-current condition at time (B) and the final over-current conditionat time (C). It is the difference in times between time (B) and time (C)which is measured to determine if an electrical short exists due to aflake. When the time of the over-current condition is sufficiently long,the decision that an electrical short due to the presence of flake (11)would be made.

As mentioned in the prior paragraph, power supply (6) usually controlsthe process in a power regulation mode. In many applications, however,other regulation modes such as voltage or even current regulation modesare utilized. Naturally, the present invention would encompass suchaspects with little variation. It should be understood that theprogressive increase in current is also useful in a process involving acurrent regulation mode since the current does overshoot and reactunstably as in other regulation modes.

Referring to FIG. 3, the effect of these varying current behaviors onthe impedance across power supply (6) can be seen. Prior to theoccurrence of flake (Il) at time (A), the impedance across power supply(6), namely that of plasma cloud (8) is in a relatively nominal state(Z_(nom)) with the inherent noise. Again, the noise shown in FIG. 3 isnot drawn to scale as it is merely meant to convey the conceptualsituation across plasma cloud (8). In addition, it should be understoodthat noise would exist throughout all phases, however, for properunderstanding, it has been removed between times (B) and (E). At time(B) an overcurrent condition occurs. It is believed that both theovercurrent condition and the subsequent shut-off of power supply (6)results in almost immediate quenching of the charged condition of plasmacloud (8) through both recombination and attraction to other surfaces inchamber (1). Although this quenching results in an increase in theimpedance of plasma cloud (8), the presence of flake (11) results in adecrease in the impedance across power supply (6). This decreasedcondition shown in FIG. 3 at time (B) will then exist so long as flakeremains. Naturally, little noise will exist in the impedance so long asflake (11) exists across gap (9).

Once the presence of an electrical short due to flake (11) is detected,the second crucial step in the methods presented is that ofautomatically removing it. While those skilled in the art had, prior tothe present invention, either broke the vacuum in chamber (1) andmanually removed flake (11), or utilized a separate power source to meltthe flake, the present invention provides methods where the conditionwould be automatically accommodated by the normal power supply. Anaspect of this general approach is to remove flake (11) throughmanipulating power supply (6).

This manipulation would involve substantially stopping the applicationof power through power supply (6). This is typically done by pulling thecurrent of power supply (6) to near a zero state. For practical reasons,current may even be pulled below zero, which through inclusion of diodesin the circuitry may appear as a short period of no current prior to theinitiation of the progressive increase. Since this delay is merely apractical expedient and is not critical to the process, it is not shownin FIG. 2. Since both plasma cloud (8) and power supply (6) havecapacitance, the nature of the sputtering process and the equipmentassociated with it makes it difficult and unnecessary to absolutelyeffect a complete stop of the application of power. Rather, asubstantial reduction, usually through pulling the current through powersupply (6) low would be sufficient as an initial step of the automaticclearing phase. Immediately upon the stop of the application of power,the removal of flake (11) is commenced through a progressive increase inthe application of power supply (6). By "progressive," any number offunctions through which power, current or any other parameter isincreased is meant to fall within the spirit and scope of the presentinvention although the practical difficulties associated with aprogressive increase in voltage or power make current the most practicalfocus at the present time. In the preferred embodiment, the circuitrydiscussed later provides for a linear increase in current through powersupply (6). Again, while other modes or functions can be chosen, alinear increase in current seems to provide not only a method which canbe easily obtained but also a method which would be stable andcontrolled in an inherently unstable environment.

Since the physical processes occurring in chamber (1) during normalsputtering are usually controlled best by regulating a constant poweracross power supply (6), such regulation mode is assumed prior to theoccurrence of any electrical short. Although known techniques to negaterecoverable arcs involve merely shutting off and delaying theapplication of a set power in such mode, they do not work for therecovery from an electrical short due to a flake. In particular, thedrastically reduced impedance across power supply (6) results in currentovershoot immediately upon the application of even a small voltageacross power supply (6). Although a maximum current protection system isfrequently incorporated, such a system rarely can accommodate theovershoot associated with the almost immediate current increase upon thereapplication of even a low voltage across power supply (6). Thus, animportant feature of the methods presented is switching modes from poweror voltage to some other mode. Because a linear increase in currentpromotes a more stable and controlled recovery, a switch to a currentregulation mode upon the occurrence of an electrical short due to flake(11) is a feature of this embodiment of the present invention. Thisfeature involves power supply (6) automatically switching to the currentregulation mode upon sensing flake (11) and pulling the current to zeroas required to substantially stop the application of power supply (6).Immediately thereafter, current through power supply (6) would belinearly increased to remove flake (11) through melting. The linearlyincreasing current begins shortly after time (C) in FIG. 2 as soon asthe current reaches the low condition. This increase continues (as shownin FIGS. 2 and 3) until either flake (11) melts or (as shown in FIGS. 4and 5) a set level is reached. Referring now to FIGS. 2 and 3, asituation where the flake melts prior to reaching the set level isshown. This represents a smaller flake than that described in FIGS. 4and 5. At time (D) it can be seen that the current rapidly decreases attime (D) when flake (11) melts. The comparison graph of impedance inFIG. 3 shows that at time (D) when flake (11) melts the impedanceimmediately goes very high. This is due to the fact not only that flake(11) no longer bridges gap (9) but also to the fact that plasma cloud(8) has been quenched and no longer exists. Since the circuitry of powersupply (6) is linearly increasing the current during this time period,the current would quickly recover and continue its linear increase. Thisrecovery results in very high voltages, thus restriking plasma cloud (8)and causing impedance to drop quickly and ultimately return to itsnominal condition (Z_(nom)). This drop in impedance is shown in FIG. 3between time (D) and (E). At time (E), the current reaches its nominalcondition (I_(nom)) and the process is returned to its prior regulationmode and continues as it existed prior to the occurrence of flake (11).Referring to FIG. 2, it can be seen that in the vicinity of time (E)there is a slight overshoot in current. This over shoot is inherent tomany power supplies and is not required for the present invention. It ismerely shown as a practical limitation of the types of power suppliesinvolved in the present invention. Again, after the current set level isreached, noise is included in both the current and impedance plots shownin FIGS. 2 and 3 to present realistic conditions.

As mentioned earlier in the context of discriminating betweenrecoverable arcs and an electrical short due to a flake, since theparameters desired may vary from process to process, so too the timerequired for the linear increase may also be optimized to the particularenvironments involved. Although it is desirable to have the increaseoccur in the shortest possible time, current overshoot as in thestrictly power-regulated situation limits how short a time might beutilized for the increase in current. Experimentally, it is deemed thatvariation in the time for increase in current from approximately 50 to550 milliseconds is a reasonable range for the particular processesinitially studied. Although the higher end, 550 milliseconds, is usedfor those specific conditions mentioned earlier, shorter periods of timeare possible.

A margin between the maximum current level of the power supply, theover-current level (I_(oc)), and the point at which the current rampstops should also be provided to accommodate any overshoot or noise thatmight occur at the top of the ramp as is inherent in many powersupplies. If flake (11) has not been melted by the time current reachesthis upper level, the current can be held for a sufficient period oftime in which to melt flake (11) if possible. This is the situationshown in FIGS. 4 and 5, that of depicting the occurrence of a largerflake. Since holding a larger current through the flake for some largerperiod of time may slowly ablate or melt the flake away for somematerials, this option may be important. In such instances, current canbe held high indefinitely until operator intervention stops the process.For other materials, it may be desirable to shut off the power supply ifremoval has not been effected in a certain period of time and even toalert the operator automatically. Naturally, each of these aspects wouldfall within the embodiments possible under the present invention.Monitoring the power output is utilized throughout to determine whenrecovery has been completed.

Referring to FIGS. 4 and 5, the above step of removing flake (11) aftercompletion of the linear increased in current can be further understood.In these figures, flake (11) occurs at time (F) and detection of itoccurs at time (G). This is similar to that shown in FIGS. 2 and 3. Atthis point, power supply (6) is automatically switched from a powerregulation mode to a current regulation mode and the current isimmediately pulled toward zero. As shown in FIG. 4, the current issubstantially zero. As mentioned earlier it is not necessary that thecurrent be pulled completely to zero but rather that it be substantiallyreduced. Upon reaching substantially a zero state, the current isprogressively increased to a current hold level (I_(h)) beginningshortly after time (G). As discussed earlier, it should be understoodthat a particular embodiment may include a brief delay aftersubstantially reducing the current. Although the present embodiment doesnot have this characteristic, it is intended that such a minordifference would fall within the scope of the present invention.

As shown in FIGS. 2 and 4, the progressive increase is linear, althoughany other function is certainly possible. Considerations that mightnecessitate more complex circuitry or programming and might not providefor the degree of control desired are of course relevant. The periodfrom time (F) to time (G) and time (G) to time (H) may also be varied oreven optimized by programming as mentioned earlier. Upon initiallyreaching the hold current (I_(h)) at time (H), it can be seen in FIG. 4that some overshoot of the current may occur. This overshoot is inherentin many power supplies and is one reason for selecting the hold level(I_(h)) at some level below the current over-current level (I_(oc)),

The current eventually stabilizes at the hold level (I_(h)) as shown attime (I). At this point the circuitry holds the current at (I_(h)),during which melting of flake (11) may continue. At a point, time (J),flake (11) melts. As before, current rapidly decreases and impedancerapidly increases. Recovery of plasma cloud (8) and reduction ofimpedance is then effected as explained earlier. The current thenreduces to the nominal current condition (I_(nom)) as it existed, priorto the occurrence of the electrical short. This is shown in FIGS. 4 and5, at time (K). At this point, recovery from the flake is complete asplasma cloud (8) has returned to the condition existing prior to theelectrical short and because the primary power supply has returned tothe power regulation mode. Although there may still occur flakes whichcannot be melted through these techniques, it is believed that such willbe relatively rare for most process conditions.

Naturally, the methods discussed herein can be accomplished in a varietyof ways ranging from hardwired circuitry to a computer implementation.Computer implementation could include programming the various steps andparameters on a programmable processor such as a microprocessor or on aseparate controlling computer. Such programming might even includeoptimization routines to automatically determine the appropriate settingfor the minimum time necessary to detect an electrical short caused by aflake or automatically determining the optimum levels and ways toprogressively increase current to remove flake (11). It is intended thatsuch variations would fall within the scope and spirit of the presentinvention as they could be easily accomplished by those skilled in theart once the methods of the present invention are understood. Althoughat present only hardwired embodiments have been achieved, the level ofknowledge of those skilled in the art is such that embodiment insoftware or microprocessor programming through its associated memorycould be easily accomplished without further explanation or undueexperimentation. Such a programmable processor could internally controlpower supply (6) or could indeed control the entire sputtering system.Again, the step of programming the processor could easily beaccomplished by those skilled in the art without undue experimentationbased upon only a disclosure of the steps mentioned herein.

The circuitry embodiment is discussed through the block diagrams shownin FIGS. 6 and 7 and the schematic shown in FIG. 8. Referring now toFIG. 6, it should be understood that a general type of power supplyconnected as power supply (6) may involve a regulated power source as iswell-known in the art and readily available. In block diagram form,standard regulated power supplies utilize commercially available ACpower (50) which may then be conditioned through rectifier circuitry(12) to convert to direct current. The output of rectifier circuitry(12) is then conditioned by filter circuitry (13). Control circuitry(14) may also be included. In the high frequency, low energy storagepower supplies to which the present invention is particularly useful,the output may then be reconverted into a higher frequency AC powerthrough inverter circuitry (51) for certain size and process-relatedconsiderations, and then passed through second rectifier circuitry (52)and second filter circuitry (53). The output from second filtercircuitry (53) is then tapped for regulation before being passed tosystem (15).

An important feature of this type of power supply is the fact that thepower supply is regulated. Although many techniques of regulation arepossible, regulation through a feedback technique is the technique thathas been utilized in the preferred embodiment. The feedback techniquetypically involves a feedback line (16) which, after passing throughfeedback signal conditioner (49), provides information concerning thevarious parameters of the output of the power supply (6). Thisinformation is then processed through regulator circuitry (17) as iswell known in the art. In addition, the desired condition may beestablished through a manual setting or programmer (54), here shown withpower setpoint line (42). This line establishes the level that regulatorcircuitry (17) attempts to maintain. In furtherance of one of theobjects of the present invention, a simple addition to regulatorcircuitry (17) is the interconnection of the preferred embodimentreferred to as flake removal circuitry (18). Flake removal circuitry(18) interacts with regulator circuitry (17) by utilizing theinformation provided by feedback line (16). This information is thenprocessed to provide the appropriate input to control circuitry (14) inorder to produce the effects described herein and achieve the methodsdiscussed earlier.

Referring now to FIG. 7, flake removal circuitry (18) is furtherdescribed in block form. From the diagram it can be seen that,conceptually, feedback line (16) provides information which is thensplit by feedback signal splitter (19). Although for simplicity andconsistency with the conceptual diagram of FIG. 6, it has been shownthat the three feedback signals derive from the same feedback line, itshould be understood that in practice the separate feedback signals maybe tapped from different points in the circuitry of the power supplyshown in FIG. 6 including inverter circuity (51) and thus need not besplit from a single feedback line (16) as shown. Particularly withrespect to the circuitry shown, it should be understood that thefeedback signals are actually tapped from such different points in thepresent embodiment and thus feedback signal splitter (19) does notexist. It has been shown only to conceptually simplify understanding thecircuitry. Over-current condition line (20) indicates the over-currentcondition and is actually accessed from the inverter circuitry (51).Current feedback line (21) indicates the level of the current output bythe power supply and is accessed from the output of power supply (6).Power feedback line (22) indicates the product of the voltage and thecurrent output by the power supply and is also accessed from feedbacksignal conditioner (49). Since such techniques and outputs are readilyavailable or easily accessed in most power supplies, the specificinterconnection for each of these lines is readily understood by thoseskilled in the art and needs no further explanation.

Referring now to over-current condition line (20), it can be seen thatthis input is provided to sensor circuitry (23) of flake removalcircuitry (18). Sensor circuitry (23) functions to discriminate betweenover-current conditions due to a recoverable arc and over-currentconditions due to a flake. This is accomplished by timing the durationof the over-current condition and thus sense the existence of anelectrical short caused by a flake as described above in reference tothe methods involved. Upon sensing the presence of a flake, sensorcircuitry (23) triggers ramp-and-hold circuitry (24) to provideappropriate output which will cause control circuitry (14) to cause thedesired response of power supply (6). This is accomplished byinterconnecting the output of ramp-and-hold circuitry (24) after passingthrough buffer (25) with the output of current feedback line (21). Forproper response, current feedback line (21) first passes through currentsignal conditioner (26) in order to achieve the proper response. Currentsignal conditioner (26) serves to both buffer and invert the signal ofcurrent feedback line (21) as required for proper combination throughfirst summing junction (27) with the buffered output of ramp-and-holdcircuitry (24). Although obvious to those skilled in the art, it shouldbe understood that first summing junction (27) combines a positive andnegative signal to create the difference between the absolute values forproper control. This first summing junction (27) serves, throughnegative feedback techniques, to combine the outputs of current feedbackline (21) and ramp-and-hold circuitry (24). Thus combined, a currentmode regulation output from first summing junction (27) is amplifiedthrough current mode error amplifier (28). This output from flakeremoval circuitry (18) is then combined with the output of well-knownpower regulation circuitry through junction (29). As shown, it can beseen that in similar fashion power regulation is achieved through apower mode error amplifier (30). Junction (29) thus functions as an "OR"combination of both the current mode regulation output from flakeremoval circuitry (18) and the power mode regulation output from powermode error amplifier (30). This "OR" combination is in essence a lowsignal select which automatically allows control to switch betweencurrent and power regulation modes very rapidly. This is accomplished byswitching control to whichever error amplifier has the lowest absolutesignal through function of diodes (43) shown in FIG. 8. Diodes (43)serve to block whichever signal line has a higher absolute amplitude.The "OR'd" output of junction (29) then passes through control signalcondition (31) to provide input to control circuitry (14) as shown inFIG. 7. Control signal condition (31) functions to both buffer andinvert the signal as required by the specific control circuity (14)utilized in the present embodiment.

Referring now to FIG. 8, the schematic details of flake removalcircuitry (18) are shown. As can be seen in the schematic over-currentcondition line (20) is actually two separate lines (32 and 33) in thepresent embodiment. Both overcurrent condition line 1 (32) andover-current condition line 2 (33) function the same, they simplyprovide two different taps from which to sense the existence of anover-current condition. This is due to the fact that the particularpower supply modified through addition of flake removal circuitry (18)in the present embodiment has two parallel components of invertercircuitry to produce sufficient current. Although ideally bothcomponents should indicate the presence of an over-current conditionsimultaneously, in practice such is not the case. The two overcurrentcondition lines (32 and 33) are necessary to sense the occurrence of anover-current condition as soon as possible. Each of the two over-currentcondition lines (32 and 33) are also filtered through operation ofcapacitors (45), diodes (44), and resistors (46). These combinationsserve to filter any transient spikes which might occur on either of theover-current condition lines (32 and 33) and thus transform the signalsinto a form which is more readily usable.

Both over-current condition line (32) and over-current condition line 2(33) input into sensor circuitry (23). Within sensor circuitry (23)over-current condition line (32) and over-current condition line 2 (33)are connected through NAND gate (34). The output from NAND gate (34) isthen connected through the circuitry shown which includes adjustablesensor resistor (35) and sensor capacitor (36). The output from thiscircuitry is then connected through gate (37) which serves as a triggerto provide output upon sensor circuitry (23) reaching the properthreshold. This threshold is established through the settings ofadjustable sensor resistor (35) and sensor capacitor (36). Sensorcircuitry (23) functions as a timer to determine the length of timeduring which an over-current condition has existed through requiring aspecific time to charge sensor capacitor (36). The time required tocharge sensor capacitor (36) is adjusted through adjustable sensorresistor (35) to establish the appropriate parameters as discussedearlier with respect to the methods of sensing the presence of anelectrical short. Output from the sensor circuitry (23) is then invertedthrough gate (47) and becomes the input to ramp-and-hold circuitry (24).Sensor circuitry (23) is also designed to allow immediate discharge ofsensor capacitor (36) if over-current condition lines (32 and 33)indicate that the over-current condition has ceased to exist. Throughthis feature, although recoverable arcs cause sensor capacitor (36) tobegin charging, recovery from them causes the immediate discharge ofsensor capacitor (36) re-initializing it to properly time the occurrenceof a electrical short almost immediately.

Ramp-and-hold circuitry (24) consists of adjustable ramp resistor (38)which varies the time for the complete ramping to occur throughconnection with op amp (39) as an integrator, as is well known in theart. Output from op amp (39) is then held at a maximum level asdetermined through adjustable hold resistor (40). Again as was discussedearlier with respect to the methods involved, the level at which theramp is held constant is determined by allowing a sufficient marginbelow the over-current level (I_(oc)) to accommodate any overshoot. Theoutput of this circuitry is then passed through buffer (25) and intofirst summing junction (27) and is combined with the current levelsignal through simple connection.

As can be seen in the schematic, current signal conditioner (26)consists of an op amp connected as shown and as is well known in theart. Output from first summing junction (27) passes through current modeerror amplifier (28) which consists of standard amplification circuitryas shown. Current mode amplifier (28) amplifies the sum of the signalsat summing junction (27). Similarly power mode error amplifier (30)amplifies the sum of the signals at second summing junction (41). Thesesignals consist of power feedback line (22) and power setpoint line(42). This output is then combined with the output of current mode erroramplifier (28) at junction (29). This output is conditioned by controlsignal conditioner (31) and becomes the input to control circuitry (14).As can be seen the entire flake removal circuitry (18) is shown in FIG.8. It should be understood that although both first- and second-summingjunctions (27 and 41) utilize techniques well known in the art, in thepresent embodiment they are using negative feedback signals. Thesenegative feedback signals are added to positive circuitry outputs tocreate a signal which is the difference between the desired level andthe feedback or actual output level to properly control power supply(6).

With respect to the specific values used in flake removal circuitry(18), the significant values are shown in FIG. 8. Naturally differentvalues might be utilized without departing from the scope and spirit ofthe present invention. Standard part designation numbers for packagedcomponents are also shown. These numbers, such as "TLO84" for buffer(25), indicate the particular part type used in the present embodiment.Naturally, other part types might also be utilized as there are manyways in which to achieve the desired goals mentioned herein. It shouldalso be understood that while the circuitry shown in FIG. 8 representsthe preferred embodiment at the present time, many variations of thecircuitry and even entirely different circuitry or software are possibleto achieve the objects of the present invention. To the extent suchvariations utilize the teaching of this present invention and achieveits methods and purposes, it is intended that such other variations willalso fall within the scope and spirit of the present invention.

We claim:
 1. A method of removing electrical shorts caused by a flake ina sputter system, comprising the steps of:a. driving the sputter systemin normal operation by a power supply; b. detecting the presence of anelectrical short caused by a flake across said power supply; and c.manipulating said power supply in a manner which melts said flake toremove the electrical short.
 2. A method of removing electrical shortsin a sputter system as described in claim 1 wherein said step ofmanipulating the power supply is automatic.
 3. A method of removingelectrical shorts in a sputter system as described in claim 2 whereinsaid step of manipulating the power supply comprises the steps of:a.substantially stopping the application of power by the power supply; andthen b. progressively increasing the application of current by the powersupply.
 4. A method of removing electrical shorts in a sputter system asdescribed in claim 2 wherein said step of driving the sputter system bya power supply comprises the step of regulating said power supply in afirst mode and wherein said step of manipulating the power supplycomprises the steps of sequentially:a. switching to regulate the currentof said power supply upon detecting the presence of an electrical short;b. substantially stopping the application of current through the powersupply; and c. progressively increasing the current output by the powersupply.
 5. A method of removing electrical shorts in a sputter system asdescribed in claim 4 and further comprising the step of returning toregulate said power supply in said first mode after progressivelyincreasing the current output by said power supply and upon the removalof the electrical short.
 6. A method of removing electrical shorts in asputter system as described in claim 4 and further comprising the stepof holding the progressive increase of said current output upon reachinga set level.
 7. A method of removing electrical shorts in a sputtersystem as described in claim 5 and further comprising the step ofholding the progressive increase of said current output upon reaching aset level prior to said step of returning to regulating said powersupply in said first mode.
 8. A method of removing electrical shorts ina sputter system as described in claim 7 and further comprising the stepof monitoring the criteria of said first mode of said power supply whileaccomplishing the step of manipulating said power supply to remove theelectrical short.
 9. A method of removing electrical shorts in a sputtersystem as described in claim 5 wherein said step of returning to saidfirst mode of regulation is accomplished when the criteria of said firstmode reaches a particular value.
 10. A method of removing electricalshorts in a sputter system as described in claim 8 wherein said step ofreturning to said first mode of regulation is accomplished when thecriteria of said first mode equals 100% of the set value of thatcriteria prior to detecting the presence of the electrical short.
 11. Amethod of removing electrical shorts in a sputter system as described inclaim 4 wherein said progressive increase is linear.
 12. A method ofremoving electrical shorts in a sputter system as described in claim 10wherein said progressive increase is linear.
 13. A method of removingelectrical shorts in a sputter system as described in claim 12 whereinthe time of said linear increase is adjustable.
 14. A method of removingelectrical shorts in a sputter system as described in claim 10 whereinthe time of said linear increase is from 50 ms to 550 ms.
 15. A methodof removing electrical shorts in a sputter system as described in claim4 wherein the time of said progressive increase is from 50 ms to 550 ms.16. A method of removing electrical shorts in a sputter system asdescribed in claim 1 wherein said step of detecting the electrical shortcomprises the step of sensing an over-current condition in said powersupply for a prescribed period of time.
 17. A method of removingelectrical shorts in a sputter system as described in claim 16 whereinsaid time for sensing the over-current condition is adjustable.
 18. Amethod of removing electrical shorts in a sputter system as described inclaim 16 wherein said electrical short is detected by sensing theover-current condition for a period of time from 10 ms to 5 seconds. 19.A method of removing electrical shorts in a sputter system as describedin claim 16 wherein said electrical short is detected by sensing theover-current condition for 50 ms.
 20. A method of removing electricalshorts in a sputter system as described in claim 5 wherein said step ofdetecting the electrical short comprises the step of sensing anover-current condition in said power supply for a prescribed period oftime.
 21. A method of removing electrical shorts in a sputter system asdescribed in claim 20 wherein said time for sensing the over-currentcondition is adjustable.
 22. A method of removing electrical shorts in asputter system as described in claim 20 wherein said electrical short isdetected by sensing the over-current condition for a period of time from10 ms to 5 seconds.
 23. A method of removing electrical shorts in asputter system as described in claim 20 wherein said electrical short isdetected by sensing the over-current condition for 50 ms.
 24. A methodof removing electrical shorts in a sputter system as described in claim13 wherein said step of detecting the electrical short comprises thestep of sensing an overcurrent condition in said power supply for aprescribed period of time.
 25. A method of removing electrical shorts ina sputter system as described in claim 24 wherein said time for sensingthe over-current condition is adjustable.
 26. A method of removingelectrical shorts in a sputter system as described in claim 24 whereinsaid electrical short is detected by sensing the over-current conditionfor a period of time from 10 ms to 5 seconds.
 27. A method of removingelectrical shorts in a sputter system as described in claim 24 whereinsaid electrical short is detected by sensing the over-current conditionfor 50 ms.
 28. A method of removing electrical shorts in a sputtersystem as described in claims 2, 3, 4, 6, 8, 9, 13, 17, 21, or 25 andfurther comprising the steps of:a. controlling said power supply througha programmable processor; and b. programming said processor toautomatically execute the sequence of steps involving the power supply.